1. Field of the Invention
The present invention relates to shunt regulation, and in particular, to an improved shunt regulating circuit associated with a low-voltage control winding on a ferroresonant transformer for regulating high-voltage output of the transformer.
2. Description of the Prior Art
A helium-neon laser typically requires 12 to 13 KV for starting. Once started, such a laser requires a constant current of approximately 6 ma at about 2200 volts for operation.
A typical power supply for operating a He-Ne laser consists of a ferroresonant transformer operated from a line frequency supply. The transformer output is fed to a voltage multiplying circuit for increasing the voltage to about 13 KV for starting the laser. The open-circuit output voltage is regulated by a standard shunt regulating circuit associated with a low-voltage control winding in the ferroresonant transformer. After starting the laser, loading of the multiplier circuit causes the output voltage to drop from 13 KV to about 2200 volts for continuing laser operation. The output voltage under load continues to be regulated by the shunt regulating circuit.
It is desirable that such a power supply operate from 50 Hz or 60 Hz and provide plus or minus 1 KV regulation with an input of plus or minus 10% of the nominal input voltage. The output of a ferroresonant transformer is a function of the line frequency of the input supply, since flux saturation of such a transformer varies inversely with frequency. Normally, for output regulation with 50 Hz and 60 Hz line frequency operation, taps are provided on the transformer primary winding or, two different transformers are used having different stack heights. Such an arrangement still will result in the output voltage varying by perhaps 25% from low-line to high-line input due to inherent characteristics of the ferroresonant transformer under no-load circumstances.
Another method to regulate an open-circuit, high-voltage output is to sense the output voltage and then feed it back to a control circuit to effect regulation. Sensing an output voltage in the range of 12 to 13 kv is not economical. Also, a feedback arrangement would load down the line frequency operated multiplier circuit when such is used to raise the high voltage output of a ferroresonant transformer, thereby lowering the multiplier output voltage, partially eliminating the main function of the multiplier.
It is desirable, therefore, to provide a means for regulating the output of a high-voltage circuit, including a ferroresonant transformer, without making use of feedback circuitry. It is further desirable that such high-voltage output be regulated typically within plus or minus 8% while the input voltage to the primary of the ferroresonant transformer varies plus or minus 10%, and indeed, on a ferroresonant transformer capable of operation from either 50 Hz or 60 Hz.